Conventionally, switching power supply circuitry with a light weight, a small size, and high power efficiency is widely used for battery-driven equipment that uses a DC battery such as a dry cell as a power supply source and includes a portable terminal such as a mobile phone and a notebook computer.
Particularly in recent years, as the number of battery-driven equipment such as electronic equipment increases, a low-voltage operation and a long-lasting operation of equipment, that is, long-life batteries are growing in demand for power supply circuitry, including this kind of switching power supply circuitry.
FIG. 11 shows a conventional example described in Japanese Patent Laid-Open No. 2005-312191 and so on. In this switching power supply circuitry, an input voltage VIN applied to an input terminal T1 is outputted as an output voltage VOUT to an output terminal T2 through a DC-DC converter 100.
In the DC-DC converter 100, a step-up converter is made up of a choke coil 5, a main switch 6 including an N-channel MOS transistor, a rectifier diode 7, a rectifier switch 8, and an output smoothing capacitor 9. The output of the rectifier diode 7 and the rectifier switch 8 is smoothed by the output smoothing capacitor 9 and supplied to a load 10 connected to the output terminal T2.
A ring oscillation circuit 11 is supplied with power from the input terminal T1 and determines an on/off period when the main switch 6 and the rectifier switch 8 are started through a driving circuit 15.
A PWM control circuit 12 is supplied with power from the output terminal T2 and determines the on/off period of the main switch 6 and the rectifier switch 8 in order to control the output voltage VOUT through a switch 14 and the driving circuit 15.
A switching control circuit 13 switches the switch 14 so as to select a driving signal from the ring oscillation circuit 11 upon startup and select a driving signal from the PWM control circuit 12 in a steady state. The driving circuit 15 drives the main switch 6 and the rectifier switch 8 in response to the driving signal selected by the switch 14.
The switching power supply circuitry is started by applying a voltage not lower than a predetermined voltage to the input terminal T1. When the input voltage VIN is low, to be specific, when an AA battery is connected to the input terminal T1 to operate the switching power supply circuitry, it is necessary to operate the circuitry with an extremely low input voltage of about 1 V.
The gate of a CMOS structure has a threshold voltage of nearly 1 V and thus there is little margin from the input voltage VIN of about 1 V. Particularly regarding a circuit like the PWM control circuit 12 requiring the function of adjusting the on/off time of the driving signal in response to an output feedback signal, simplification for a low power supply voltage operation is limited, making it difficult to design the circuit. In other words, it is difficult to perform temperature compensation for a circuit operation and reduce variations in characteristics, making it difficult to stably control the circuit operation. In the switching power supply circuitry of FIG. 11, the main switch 6 and the rectifier switch 8 are turned on/off upon startup in response to the driving signal from the ring oscillation circuit 11 supplied with power from the input voltage VIN, and the voltage is increased without feedback control on the output voltage VOUT.
In the ring oscillation circuit 11, inverters of multiple stages are connected like a ring, so that the ring oscillation circuit 11 can be operated with a low power supply voltage. In a CMOS structure, it is only necessary to have a power supply voltage slightly higher than the gate threshold voltage. In other words, an oscillation can be obtained by a power supply voltage of no more than 1 V.
To be specific, it is detected that the output voltage VOUT is low and the switching control circuit 13 starts operating, and the switching control circuit 13 switches the switch 14 to select the driving signal from the ring oscillation circuit 11. In response to the on/off operation of the main switch 6 and the rectifier switch 8, power is transmitted from the input voltage VIN to the output voltage VOUT through the choke coil 5 and thus the output voltage VOUT increases. At this point, the switching power supply circuitry does not perform a feedback operation for controlling the output. In this way, a non-feedback boosting operation (hereinafter, will be referred to as “startup oscillation”) is performed by ring oscillation.
When the potential of the output voltage VOUT reaches a predetermined potential set by the switching control circuit 13, the switching control circuit 13 switches the switch 14 so as to select the driving signal from the PWM control circuit 12. Thus the operation of the switching power supply circuitry shown in FIG. 11 switches from “startup oscillation” to a feedback switching operation (hereinafter, will be referred to as “main oscillation”) using the PWM control circuit 12 supplied with power from the boosted output voltage VOUT. In the main oscillation, the switching power supply circuitry operates such that the output voltage VOUT reaches a target voltage.
FIG. 12 shows another conventional switching power supply circuitry.
The switching power supply circuitry is multi-channel control switching power supply circuitry in which an input voltage VIN applied to an input terminal Ti passes through a DC-DC converter 100 to output a plurality of output voltages VOUT1, VOUT2, VOUT3, . . . from output terminals T2, T3, T4, . . . . The switching power supply circuitry includes a circuit for outputting the output voltage VOUT1 (hereinafter, will be abbreviated to CH1), a circuit for outputting the output voltage VOUT2 (hereinafter, will be abbreviated to CH2), and a circuit for outputting the output voltage VOUT3 (hereinafter, will be abbreviated to CH3).
The basic configuration and operation of CH1 are similar to those of the switching power supply circuitry of FIG. 11 and thus the explanation thereof is omitted. CH2 is made up of a CH2 control circuit 16 and a step-down converter including a main switch 17, a rectifier diode 18, a choke coil 19, and an output smoothing capacitor 20. CH3 is made up of a CH3 control circuit 21 and a step-up converter including a choke coil 22, a main switch 23, a rectifier diode 24, and an output smoothing capacitor 25.
The switching power supply circuitry of FIG. 12 is also started by applying a voltage not lower than a predetermined voltage to the input terminal T1. As described in the switching power supply circuitry of FIG. 11, the CH2 control circuit 16 and the CH3 control circuit 21 are self-biased so as to be supplied with power from the output voltage VOUT1 of CH1, on fear that the control becomes unstable when the input voltage VIN is low.
For this reason, on the channels (for example, CH2 and CH3) other than CH1, switching power supply control is started when the power supply voltage of each control circuit reaches a potential enabling a stable operation, that is, after CH1 changes from startup oscillation to main oscillation.